Method and apparatus for recovering the energy of expansion of wet dusty gases under pressure

ABSTRACT

For recovering energy from a dust-laden wet gas under pressure such as blast-furnace gas, the degree of saturation of the gas is raised in a first stage by water evaporation, and in a second stage the gas is expanded through a turbine while its degree of saturation is at least equal to that obtained after the first stage.

United States Patent [191 Boudard et al.

METHOD AND APPARATUS FOR RECOVERING THE ENERGY OF EXPANSION OF WET DUSTY GASES UNDER PRESSURE Inventors: Jean Boudard, Florange; Louis Marcellin, Marcq-en-Bareuil, both of France Wendel-Sidelor and Societe DAerodynamique et de Thermodynamique Francaise, Boulogne, France Filed: Feb. 16, 1972 Appl. No.: 226,847

Assignee:

Foreign Application Priority Data Feb. 17, 1971 France 71.5402

US. Cl 60/64, 60/3905 Int. Cl. Flk 7/34, FOlk 7/22, F22d 11/00 Field of Search 60/64, 1, 39.05, 39.53,

References Cited UNITED STATES PATENTS 9/1916 Dinsmore 60/3905 June 25, 1974 2,660,521 11/1953 Teichmann 60/3905 2,667,235 /1954 Secord 60/3905 3,428,117 2/1969 Woodhead 122/7 A 3,613,333 /1971 Gardenier /89 3,683,626 8/1972 Merrill 55/84 FOREIGN PATENTS OR APPLICATIONS 2,511 1/1914 Great Britain /3955 877,046 9/1961 Great Britain 122/7 A Primary Examiner-Edgar W. Geoghegan Assistant ExaminerH. Burks, Sr. Attorney, Agent, or FirmWil1iam J. Daniel [57] ABSTRACT For recovering energy from a dust-laden wet gas under pressure such as blast-furnace gas, the degree of saturation of the gas is raised in a first stage by water evaporation, and in a second stage the gas is expanded through a turbine while its degree of saturation is at least equal to that obtained after the first stage.

3 Claims, 2 Drawing Figures METHOD AND APPARATUS FOR RECOVERING THE ENERGY OF EXPANSION OF WET DUSTY GASES UNDER PRESSURE The present invention relates to a method and apparatus for recovering part of the energy of a wet dusty gas under pressure by expanding this gas through a turbine, and more particularly to the recovery of the expansion energy of blast-furnace gas.

Modern blast-furnaces are operated in such a way that the gas leaves the throat at a pressure which may attain several kilograms per square centimetre in some cases. This hot, damp and dusty gas must be expanded and dedusted before it is utilized (usually at a pressure slightly above atmospheric pressure). This process of expansion can take place with or without mechanical work being done. The expansion process involving no mechanical work at present the most widely used offers the advantage of being technologically simpler but has the disadvantage of wasting a considerable amount of energy. Expansion in which mechanical work is done involving the use of a turbine to recover the expansion energy of the gas is an attractive notion in itself but presents certain difficulties and requires that certain conditions be met before it can be carried into practice.

Already known is a method of expanding blastfurnace gas through an axial turbine after it has been thoroughly scrubbed by a wet process. In this method, the clean, damp and relatively cool gas is heated either by partial internal combustion or by heat-exchangers so as to ensure that its temperature may in no case reach dew-point in the course of expansion in order to avoid condensation which would cause the turbine to be abraded by water droplets and fouled by wet dust deposits. The non-saturated gas leaving the turbine at a temperature of about 45 to 70 C must be scrubbed complementarily in order to rid of residual dust and lower its temperature for subsequent use. The external energy input required for a heat-exchanger, or the consumption of part of the gas with or without a reduction in its calorific value reduces much of the economic advantages of such a recovery process. Attempts have been made to suppress "the drawbacks of this method by expanding the gas after dry-scrubbing, only without wet scrubbing, the object being to avoid having to heat it or lower its sensible heat; in such cases, however, the expansion turbine is supplied with gas which is still comparatively heavily dust-laden, and furthermore the gas must undergo a secondary scrubbing operation subsequent to expansion. In another known method, the dry-scrubbed gas is expanded through a centripetal expansion turbine and is if necessary heated prior to expansion in order to avoid condensation in the turbine. This method, which is consequently advantageous only if the gas temperature is high, likewise requires a secondary scrubbing of the expanded gas before the latter is used.

The necessity in both the above-mentioned methods of terminating the expansion at a temperature which is invariably higher than the dew-point requires that the temperature of the gas on entry into the turbine be of the order of 150 C.

While a temperature" of this order is normally ob- 6 tained with certain loads, conditions nevertheless arise in which the blast-fumace throat temperatures are found to be too low to make it possible to dispense with reheating the gas prior to expansion, for instance with blast-fumaces fed with medium-grade iron ore or blown with oxygen-enriched blast in conjunction with large an injection rate of fuel oil, in which the gas temperature on exit from the blast-fumace throat may be as low as C approximately.

In the latter case, both known methods are either inapplicable or unprofitable.

The present invention provides for recovering the expansion energy of the blast furnace gas under optimum economic conditions, without heating the gas or lowering its calorific value.

In order to allow taking advantage of the relative heating of the gas due to condensation of part of its water content as the gas is expanded through a turbine, avoiding any danger of icing, and ensuring sufficient water condensation to permit effective scrubbing of the dust in the turbine, it is the object of the present invention to accomplish:

in an initial stage, an increase in the degree of saturation of the gas in order to bring the same to a state of saturation such as to ensure that no icing occur during expansion of the gas through the turbine and that the amount of water condensed will be sufficient to efficiently eliminate the dust deposits;

in a second stage, expansion of this gas with at least the degree of saturation obtained during the first stage, without any reheating of the gas between the first and second stages; the degree of saturation of the gas in the second stage can be higher than in the first stage due to a possible cooling of the gas between the two stages, but in no case lower as the gas is not reheated;

preferably, saturation of the gas during the first stage, prior to its expansion, and during the second stage its expansion in this saturated stage through a turbine, to take maximum advantage of the relative heating of the gas, particularly in the case of a gas at relatively low temperature, and to ensure abundant condensation during the expansion and hence proper washing of the dust which deposits in the turbine;

saturation of the gas during its wet scrubbing;

an installation for carrying the invention into practice, comprising:

means for introducing water into the gas;

a preferably centripetal expansion turbine coupled to means for using its mechanical energy;

means for circulating the gas;

a closed heat-insulated water circuit including means for recovering, in the means for introducing water into the gas, the dust-laden non-evaporated excess water and, at the turbine exit end, the dust-laden condensation water, means for purifying said waters comprising one or more settling basins, means for the disposal of the recovered dust, means for recirculating said waters to the means for introducing water into the gas, and means for making up water losses;

an alternative installation embodiment, in which the means for introducing water into the gas are a webscrubber.

The description which follows with reference to the accompanying non-limitative exemplary drawing will give a clear understanding of how the invention can be carried into practice.

In the drawing:

FIG. 1 is a diagram of apparatus for performing the method according to this invention; and

FIG. 2 illustrates the energy recovery principle according to the subject method of this invention, on the basis of graphs for wet blast-furnace gases.

Said method is applicable to any wet and dusty gas issuing from a reactor operating under pressure.

The subject method of this invention allows recovery, without preliminary heating, of part of the energy of the gas, to wit its expansion energy, without any danger of fouling the turbine with wet dust deposits or through icing of the water contained in the gas, and obtaining on exit from the turbine a gas which can be used immediately without any dedusting or cooling.

In accordance with said method water is introduced into the gas, after the same leaves the reactor, to bring it to a state of near or even complete saturation with water vapour before it enters the turbine.

The gas is then channelled into an expansion turbine, and the ensuing expansion causes the gas temperature to drop and to thereby produce condensation of a large part of the water vapour contained in the gas. This abundant water condensation entrains the residual dust in the gas and effectively cleanses the turbine and avoids any risk of fouling thereof.

Further, the heat provided by this condensation avoids any danger of icing within the turbine or at the exit therefrom and reduces the temperature drop caused by the expansion.

On chart 1 of FIG. 2 the abscissa or X-axis represents the temperature and the ordinate or Y-axis represents the expansion ratio R,,.

This chart gives the temperature of a gas, assumed to be dry, subsequent to a monophase expansion assumed to be such as to preclude condensation of any water vapour it may contain. The chart shows a set of isothermic solid-line curves each of which corresponds to a turbine intake temperature (T T T or T In one example, the expansion ratio is 2.5, represented by the point A. To it corresponds a point B on the isothermic temperature curve T, 60 C). The end of monophase expansion would correspond to an exit temperature T represented by the point C, which temperature is as a rule below zero.

Chart 2 on the righthand side of FIG. 2 represents a set of blast-furnace gas saturation curves. Shown on this chart, which is juxtaposed to Chart 1, are the following:

the gas temperature along the same X-axis and on the same scale as Chart 1;

the water content of the gas along the Y-axis;

the isobaric gas saturation curves (P to P shown in solid lines;

and, possibly, gas isoenthalpy curves E, to E (This second set of curves is shown in dash lines and may be taken to consist of parallel straight lines running downwardly to the right).

The point D on Chart 2 corresponds to the temperature T, 60 C) and to a normal water content J 65 grams per cubic metre.

If condensation-free expansion occurs, the representative point of the gas will shift from D to E, there being subsaturation with a constant water content.

The point E tends to be reached in a very short time, but condensation takes place with a certain time-lag. Hence point E is in practice not attained, so that the imaginary point describes the curved path 11 and reaches F, on the isobaric curve corresponding to the turbine exit pressure, by following an isoenthalpy curve (at constant heat). On the curve EF, a temperature increment dT occurs for each gram of water condensed dE.

There is thereby determined the final point F, the temperature T, of which is higher than T and even higher then 0 C, thereby avoiding icing in the turbine.

FI-l, which is the distance of F from the horizontal passing through D, represents the quantity of water condensed during the expansion, and FG, the ordinate of F, the quantity of water remaining in the gas subsequent to expansion in the vapour state.

From the isoenthalpy curves it is possible to determine the difference in enthalpy ab between the initial state at D and the final state at F, which, after correction for overall efficiency, consequently represents the energy which can be recovered during expansion.

After its expansion the gas leaving the turbine is at a temperature and has a dust content which are both low enough to make it directly usable.

The expansion turbine is coupled to means for recovering the mechanical energy it furnishes.

The description which follows of an application of the above-described method to blast-furnace gas, with reference to FIG. 1, will give a clearer understanding of how the invention can be carried into practice.

The blower 11 supplies the blast requires by blastfurnace l, flow being adjusted by setting device 12.

In a first stage, the water is introduced into the gas after exit thereof from blast-furnace 1 and passage through the dust-catcher.

The water introduction process is so regulated that the gas is in a stage of near-saturation of saturation prior to entering the turbine, that is at a temperature of a little higher than or equal to the dew-point.

It may be of particular interest to take advantage of a wet scrubbing of the gas as a means of introducing water. Accordingly, recourse is had to a wet scrubber 2 and the scrubbing is so regulated that the gas emerges partially dedusted, virtually saturated, and at a temperature a little higher than or equal to the dew-point temperature.

In a second stage, a fraction of the gas is expanded directly via the by-pass 15, controlled by the pressure regulator or septum valve 7 which, in the customary way, regulates the back-pressure at the throat of blastfurnace 1.

The bulk of the gas is supplied directly to expansion turbine 8 via a regulating valve 9. The expansion turbine is coupled to means 20 for recovering the mechanical energy furnished by the expansion of the gas.

By virtue of the reduction in enthalpy of the gas during the expansion process, an abundant condensation of water takes place in the turbine. This water entrains a large part of the dust remaining in the gas after the wet scrubbing.

Since the gas is virtually saturated upon entry into the turbine, the quantity of water which condenses permits a final gas scrubbing adequate to allow direct subsequent utilization, of the gas, as well as abundant and permanent washing of the turbine, thereby avoiding any danger of fouling through wet dust deposits.

As well known per se, an atomizer could possibly be provided at the turbine intake as a safety measure in the event of said condensation not proving sufficient, for any reason whatsoever, to ensure efficient cleansing of the turbine. This atomizer could be used to increase the amount of water used for such cleansing.

The heat released by the condensation of the water during expansion is transferred to the expanding gas which enables the drop of the gas temperature to be reduced and avoids any risk of icing in the turbine or upon exit therefrom.

Similarly, the pneumatic energy of the water vapour contained in the gas is reduced during the condensation and is recovered together with the energy produced by the expansion of the gas.

The dust-laden condensation water collected in the turbine is dispatched via conduit to a settling basin 4. Deconcentration of the water is produced by a bleed 17 that discharges all the dust collected. The water is drawn off from basin 4 by a pump 3 and is used to feed the wet scrubber 2 through conduits 5, 6. In particular, if the turbine is associated to a prior water-atomizer, the latter is fed by the same pump 3 (via 18). The cleaning-water system therefore operates virtually in a closed circuit. Make-up water is supplied at 14 to compensate for water lost with the wet dust eliminated through bleed l7 and with the saturated gas leaving the installation.

The passages of the gas through wet scrubber 2 produces a drop in temperature due to the evaporation of the water added to saturate the gas. This does not result in a loss of heat but merely in a heat transfer process between the gas and the water evaporated in this way. This temperature drop must be as small aspossible so that the amount of water vapour contained in the gas is sufficient for condensation to occur when the expansion takes place but not so large as to produce temperature low enough to cause an icing hazard. For this reason the heat losses of the overall scrubbingwater circuit must be reduced.

Advantage can then be taken of the heat contained in the water used to dedust the blast-furnace gas, part at least of this water being tapped at a temperature level making it suitable for use. It will readily be appreciated from the foregoing that the subject method of this invention is particularly advantageous.

The first stage referred to precedingly associates the saturation of the gas with its wet-scrubbing.

Dedusting by the wet process permits finer scrubbing than dedusting by the dry process and substantially lowers the temperature of the gas while increasing its relative humidity, which made it difficult to apply in the prior art methods.

The second stage of the process, namely the expansion of a wet gas at relatively low temperature, exerts a dedusting action without involving additional means. The condensation which occurs during the expansion assists efficient operation of the turbine and hence the energy recovery process.

The secondary scrubbing which takes place at the same time as the energy recovery process can therefore be considered inexpensive, and the exiting gas is directly usable without having to be scrubbedanew. A wet scrubbing process designed to increase therelative humidity of the gas, if associated to a turbine designed to recover the expansion energy, will therefore suffice for scrubbing the gas in satisfactory fashion.

It is important to note that the method according to this invention applies in particular to a gas whose temperature already relatively low on exit from the blast-furnace throat is further lowered prior to entry into the turbine because of the gas having been scrubbed by the wet process. It is unnecessary to heat the gas either by internal combustion or by passing it through a heat exchanger. Hence there is neither a reduction in the calorific value of the gas nor any external energy input. The optimum gas temperature prior to entry into the turbine is approximately 60 C.

Despite this low temperature there is no risk of fouling the turbine, either internally or at its outlet, with dust or ice. In particular, in cases where the blastfumace gas is to be used, any variation in the temperature of the blast-furnace gas will have no detrimental effect on operation of the turbine.

The table below gives an example of a specific application of the method according to this invention, consisting in recovering the expansion energy of a gas from a blast-furnace operating with back pressure at the throat.

BEFORE WET SCRUBBING Gas output 350,000 Nm lhr Pressure 2 kg/cm", effective Temperature C Water content 33 g/Nm Dust content 10 g/Nm" AFTER WET SCRUBBING, AHEAD OF TURBINE Pressure 1.85 kg/cm, effective Temperature 60 C Water content Saturated gas Dust content Approximately 200 mg/Nm PAST THE TURBINE Pressure 0.2 kg/cm effective Temperature 34.5 C Water content Saturated gas Dust content 3 to 10 mg/Nm.

The energy recovered during the expansion of this gas is approximately 66 kj per kilogram of gas, or approximately 8370 kW for a flow rate of 350,000 Nm /hr.

Moreover, the subject method of this invention requires no external energy input and does not diminish the internal calorific value of the gas.

We claim:

1. In a method for treating pressurized dust-laden blast furnace gas normally having a water content below the saturation point to recover expansion energy in a turbine coupled to means for using the energy thus recovered, the improvement in which:

in a first stage said furnace gas is subjected to wetscrubbing and in the course thereof the water content of the gas is increased through evaporation of such water;

and in a second stage the wet-scrubbed gas is introduced into said turbine with the water content thereof maintained at at least the increased level obtained in said first stage.

2. A method according to claim 1 wherein:

in said first stage the water content of said gas is increased to substantially the saturation point;

and in said second stage, the gas is introduced into said turbine in said substantially saturated condition.

- 3. An apparatus for treating pressurized dust-laden blast furnace gas normally having a water content below the saturation point to recover expansion energy in a turbine coupled to means for using the energy thus 8 evaporated dust-laden condensation water, means for purifying said water and for eliminating the recovered dust, means for recirculating purified water from said purifying means, and means for adding fresh water to compensate for water losses; and means for discharging said expanded gas from said turbine.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Inventor) Jean BOUDARD et a1 It is certified that error appeers in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 7, Claim' 3, lines 1 & 2, change "centrifugal" to centripetal Signed and sealed this 24th day of September 1974.

(SEAL) Attest: McCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Commissioner of Patents USCOMM-DC 60378P69 INTING OFFICE llll O--Ill-3S4 U.$. GOVIINIIENT PR FORM PO-IOSO (10-69) 

1. In a method for treating pressurized dust-laden blast furnace gas normally having a water content below the saturation point to recover expansion energy in a turbine coupled to means for using the energy thus recovered, the improvement in which: in a first stage said furnace gas is subjected to wet-scrubbing and in the course thereof the water content of the gas is increased through evaporation of such water; and in a second stage the wet-scrubbed gas is introduced into said turbine with the water content thereof maintained at at least the increased level obtained in said first stage.
 2. A method according to claim 1 wherein: in said first stage the water content of said gas is increased to substantially the saturation point; and in said second stage, the gas is introduced into said turbine in said substantially saturated condition.
 3. An apparatus for treating pressurized dust-laden blast furnace gas normally having a water content below the saturation point to recover expansion energy in a turbine coupled to means for using the energy thus recovered, comprising an expansion turbine of the centrifugal type coupled with means for using the mechanical energy of the turbine; a gas wet-scrubbing system; means for circulating the gas from said furnace to said wet-scrubbing system; a heat-insulated water loop circuit including means in said scrubbing system for contacting said gas with water and evaporating a portion of such water into the gas, means for recovering non-evaporated dust-laden condensation water, means for purifying said water and for eliminating the recovered dust, means for recirculating purified water from said purifying means, and means for adding fresh water to compensate for water losses; and means for discharging said expanded gas from said turbine. 